For autocompletion you can redirect projects --completion to a file in
~/.bash_completion (zsh is also supported) or use projects-modified.sh
which also contains a fallback to _filedir so that you can still complete
files and directories (I've opened an
issue to get that improvement back
into omelette, the completion library we use).

Projects is primarily a framework for making it easy to execute actions on one
or more of your projects. For example, you could write a command to check the
clean/dirty status of all of your checked out git repositories and list the
dirty ones.

If you have an executable file in your PATH that starts with projects- then
you can execute it underneath projects (and you're encouraged to share them
with others!)